I spoke with Lars today for the first time in six weeks. He has already raided Fairbanks dumpsters, and he answered the phone while salvaging a motor from a discarded washing machine. It will probably end up driving his lathe. He's also building a steel work table, a slab cutting rig, a copy of my gear tooth cutter, and his mobile drill press. It would take me months to do all that stuff, but, knowing Lars, he'll probably be looking for new projects by Wednesday.
I finally found a place that sells an adjustable spade bit that goes up to a 5 inch diameter, but at $169 it probably isn't any more cost effective than a handful of large forstner bits. Lars is going to see if he can improve on my design (I'm sure he can) and make a few custom spade bits instead.